40 



PBOTOZOA 



E. lima (Ehrenberg) (Fig. 54). Anterior border of both shells 

 slightly indented; length .04 mm.; slow of movement: at Woods Hole. 



Order 2. DINIFERIDA. 



Two grooves present, a transverse and a longitudinal: 2 families. 



Key to the families of Diniferida: 



Gi Transverse groove near middle of body 1. Peridinidae 



ffa Transverse groove above the middle 2. DiNOPiiYsmAE 



Family 1. PEEIDINIDAE. 



Transverse groove medium; longitudinal groove short; shell, when 

 present, composed of plates or not; plates either equatorial (bordering 

 the transverse furrow), apical, or antiapical, while a rhombic plate may 

 extend from the transverse furrow to the apex: about 4 genera. 



Key to the genera of Peridinidae : 

 Oi With shell. 

 &i Reticular markings on shell. 

 Ci Anterior part of shell with 7 equatorial and 1 rhombic plates. 



1. Peridinium 

 Co Anterior part of shell with 3 equatorial and no rhombic plates. 



2. Ceratium 



62 No markings on shell 3. Glenodinium 



a. Without shell 4. Gymnodinium 



1. Peridinium* Ehrenberg. Body globular or elongate; shell with 



distinct transverse groove, which may be spiral and with about 20 



plates: 9 species, fresh and salt water; many species are reddish in 



color and may be in sufficient numbers to color the sea. 



P. digitale Pouchet (Fig. 55). Shell with large 



pits and with oblique furrow, 1 posterior and 2 



anterior spines; length .06 mm.: Woods 



Hole; marine; common. 



P. divergens Ehr. Shell spherical, 



tapering posteriorly, with 2 large spines 



anteriorly ; length .07 mm. : Woods Hole ; 



marine; common. 



2. CERATiUMf Schrank. Body a 



flattened sphere with 3 long projections; 



FiR. uA—E.ruvicUa transverse groove either spiral or cir- 

 lima (Calkins). ^ ^ 



cular; longitudinal groove usually wide; 



shell reticulate or striped and composed of 10 plates; color usually green 



or brown: numerous species; in fresh and salt water. 



* See "Peridinium and the 'Red Water' in Narragansett Bay," by A. D. Mead, 

 Scl. N. S., Vol. 8, p. 707, 1898. 



t See "Mutations in Ceratium," by C. A. Kofoid, Bull. Mus. Comp. Zool., Vol. 52, 

 p. 213, 1909. 



Fig. 55 



Peridinium 



digitale 

 (Calkins). 



